Evaluating greenhouse gas emissions from hydropower complexes on large rivers in Eastern Washington

Water bodies, such as freshwater lakes, are known to be net emitters of nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4). In recent years, significant greenhouse gas (GHG) emissions from tropical, boreal, and mid-latitude reservoirs have been reported. At a time when hydropower is increasing worldwide, better understanding of seasonal and regional variation in GHG emissions is needed in order to develop a predictive understanding of such fluxes within man-made impoundments. We examined power-producing dam complexes in Eastern Washington on the Snake and Columbia Rivers by sampling tributary, mainstem, embayment, forebay, and tailrace areas for N2O, CH4, and CO2 during winter and summer, 2012. At each sampling location, GHG measurement pathways included surface gas flux, dissolved gases within the surface water column, ebullition within shallow embayments, and direct sampling of hyporheic pore-water. Measurements were also carried out in a free-flowing reach of the Columbia River to estimate net GHG emissions from hydropower. Emissions of N2O and CH4 were greatest within embayments, ranging up to 6.8 mg/l and 78 mg/l, respectively. Carbon dioxide tended to be greater in embayments and in forebay environments of the hydroelectric projects, exceeding 1800 mg/l and 5,900 mg/l in these areas, respectively. Concentrations of N2O and CH4 tended to be greatest in samples that were collected directly from hyporheic pore-water, while CO2 was most prevalent within the surface water column.